Control device for direct injection engine

ABSTRACT

To reduce the amounts of HC, NOx and other emissions from a direct injection engine when a catalyst is still in its unheated state, and to promote catalyst quick light-off operation by increasing the temperature of exhaust gases, a control device comprises a temperature state identifier ( 31 ) for judging the temperature state of a catalyst ( 22 ) and a fuel injection controller ( 33 ) for controlling fuel injection from an injector ( 11 ). The fuel injection controller ( 33 ) controls the injector ( 11 ) based on judgment results of the temperature state identifier ( 31 ) in such a way that the injector ( 11 ) makes at least two-step split injection during a period from an intake stroke to an ignition point including a later injection cycle performed in a middle portion of a compression stroke or later and an earlier injection cycle performed prior to the later injection cycle at least in a low-load range of the engine when the catalyst ( 22 ) is in its unheated state, in which its temperature is lower than its activation temperature, and either of the later injection cycle and earlier injection cycle injects fuel which contributes to main combustion during a main combustion period in which approximately 10% to 90% by mass of the injected fuel is burnt in a combustion process occurring in the combustion chamber.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a control device for a direct injection engineprovided with an injector which injects fuel directly into a combustionchamber.

BACKGROUND ART

A direct injection engine having an injector for injecting fuel directlyinto a combustion chamber is conventionally known. This engine isoperated such that a condition in which a mixture is locally distributedaround a spark plug is produced by injecting the fuel in a latter halfof a compression stroke to perform so-called stratified chargecombustion in a low-load state. This operation makes it possible toincrease the air-fuel ratio (leaner mixture) while maintainingcombustion stability and improve fuel economy.

Exhaust gases from engines of motor vehicles, for instance, containhydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), andthere exists a demand today to reduce generation and release of theseharmful constituents as much as possible to achieve improved propertiesof these emissions. One approach that has conventionally been taken isto provide a catalyst in an exhaust passage, and it is a common practicein the aforementioned direct injection engine as well to provide acatalyst in its exhaust passage. A generally known example of suchcatalyst is a three-way catalyst which has the ability to clean out HC,CO and NOx approximately at the stoichiometric air-fuel ratio. Anotherexample that has already been developed is a catalyst which can cleanout NOx even in a “lean” operating range in order to be suited to leanburn operation by stratified charge combustion in the aforementioneddirect injection engine or else.

A fuel injection control device disclosed in Japanese Unexamined PatentPublication No. 4-231645, for example, is known as a device forachieving an improvement in conversion efficiency of a catalyst at lowtemperatures, for instance, in this type of direct injection engine. Ina direct injection engine having a lean NOx catalyst provided in anexhaust passage, the lean NOx catalyst being of a type that requires HCfor the reduction of NOx, this device is so arranged as to make primaryinjection from an injector in a latter part of a compression stroke, andmake secondary injection in addition to the aforementioned primaryinjection to inject a small amount of fuel for supplying HC to the leanNOx catalyst within a period from an intake stroke to an early part ofthe compression stroke when the temperature of the catalyst is low, ormake the aforementioned secondary injection in addition to theaforementioned primary injection within a period from a latter half of acombustion stroke to an early part of an exhaust stroke when thetemperature of the catalyst is high. In this device, HC derived from thefuel injected by the secondary injection is supplied to the catalyst inthe exhaust passage by setting the amount of fuel injected by thesecondary injection to such a small level that will scarcely affectcombustion within a combustion chamber, and a low boiling-pointconstituent of EC is supplied to the catalyst in low-temperatureconditions and a high boiling-point constituent of HC is supplied to thecatalyst in high-temperature conditions by varying the timing ofsecondary injection in low-temperature and high-temperature conditionsin the aforementioned manner.

A catalyst for exhaust gas conversion can not fully exhibit itsconversion effects when the catalyst is not heated yet and the catalysttemperature is lower than its activation temperature. HC and NOx arelikely to be released in large quantities in such a case. For thisreason, it is required to reduce the amounts of HC and NOx emissionsoutput from the engine to the exhaust passage and to promote catalystquick light-off operation by increasing the exhaust gas temperaturewhile the catalyst is still in its unheated state, for instance.

The aforementioned direct injection engine, however, has a problem thatits catalyst is not easily heated because the amount of heat releasedinto the exhaust passage is small in a case where the air-fuel ratio isincreased by performing stratified charge combustion through injectionin the compression stroke even when the catalyst is not heated yet.

Although the device disclosed in the aforementioned Publication isintended to achieve an improvement in low-temperature catalystperformance by making, in addition to the primary injection during thelatter part of the compression stroke, the secondary injection precedingthe primary injection when the temperature of the catalyst is low, theamount of fuel injected by this secondary injection is extremely smalland is delivered to the catalyst in the exhaust passage almost withoutburning within the combustion chamber. Therefore, this device isadvantageous only when a lean NOx catalyst of a type that requires HCfor the reduction of NOx is used. Moreover, the device makes it possibleto achieve NOx conversion effects with the supply of HC only after thecatalyst has been activated to a certain degree, though it is still in alow-temperature state, and because HC is released in an earlier unheatedstate than that point, the device is not favorably suited for achievingan improvement in emissions. Furthermore, the device does not have thefunction of promoting the catalyst quick light-off operation by anincrease in the exhaust gas temperature.

Another measure to deal with cold start of a direct injection engine is,as disclosed in Japanese Unexamined Patent Publication No. 4-187841,such that ignitability is maintained by increasing the amount of fuelinjected during the compression stroke while the internal temperature ofcylinders is low. More specifically, the engine is controlled to makeinjection during the compression stroke in a low-load range, splitinjection during the successive intake and compression strokes in amedium-load range and injection during the intake stroke in a high-loadrange when the engine is in its warm-running condition, whereas theaforementioned range of split injection is extended to the high-loadside while the engine is still cold.

This device, however, maintains the ignitability simply by increasingthe amount of fuel injected in the compression stroke by as much as anamount corresponding to deterioration of evaporation and atomizationwhile the engine is cold, and the device does not have the ability toaccomplish quick light-off and emission improvement by an increase inthe exhaust gas temperature while the catalyst is still in its unheatedstate.

In the light of the above-described circumstances, it is an object ofthis invention to provide a direct injection engine capable of reducingthe amounts of emissions, such as HC and NOx, from the engine andpromoting light-off of a catalyst by increasing the exhaust gastemperature while the catalyst is still in its unheated state, forinstance, so that the emissions are significantly achieved through areduction in the time required until the catalyst is brought to itsunheated state and a reduction in the amounts of HC and other emissionswhen the catalyst is not heated yet.

DISCLOSURE OF THE INVENTION

According to the invention, a control device for a direct injectionengine controls the engine in such a way that it makes at least two-stepsplit injection during a period from an intake stroke to an ignitionpoint including a later injection cycle performed in a middle portion ofa compression stroke or later and an earlier injection cycle performedprior to the later injection cycle at least in a low-load range of theengine when a catalyst is in its unheated state, in which itstemperature is lower than its activation temperature, and that either ofthe later injection cycle and earlier injection cycle injects fuel whichcontributes to main combustion during a main combustion period. Thiscontrol operation ensures ignitability and combustion stability afterignition and produces slow burning in a latter part of a combustionperiod when the split injection is conducted with the catalyst in itsunheated state. It is therefore possible to reduce HC and NOx in exhaustgases released from a combustion chamber when the catalyst is still inits unheated state, resulting in an improvement in emissions, andsignificantly promote catalyst quick light-off operation as a result ofan increase in exhaust gas temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general configuration diagram showing a direct injectionengine according to an embodiment of the invention;

FIG. 2 is a diagram showing injection points of earlier injection andlater injection in split injection;

FIG. 3 is a time chart showing an example of control operation;

FIG. 4 is a diagram showing an output of an O₂ sensor;

FIG. 5 is a diagram showing variations in the output of the O₂ sensorobserved when feedback control of the air-fuel ratio is performed andcorresponding variations in a feedback correction coefficient;

FIGS. 6(a)-6(c) are diagrams showing variations of pistons for thedirect injection engine;

FIG. 7 is a diagram showing variations in the proportion by mass ofcombusted fuel observed when split injection was conducted and whenone-time injection was conducted in an intake stroke;

FIG. 8 is a diagram showing variations in flame front area observed whensplit injection and one-time injection were conducted;

FIG. 9 is a diagram showing exhaust gas temperature and fuel economyrate observed when later injection timing is altered in various wayswhile performing split injection and when the amount of ignition timingretardation is altered in various ways while performing one-timeinjection;

FIG. 10 presents graphs showing (a) exhaust gas temperature, (b) HCconcentration, (c) NOx concentration and (d) engine speed fluctuationcoefficient obtained when split injection and one-time injection wereconducted;

FIG. 11 is a diagram showing variations with time of HC reduction rate,NOx reduction rate, exhaust gas temperature and vehicle running speed ona vehicle-mounted engine;

FIG. 12 is a diagram showing the relationship between the ignitiontiming and indicated mean effective pressure when intake stroke one-timeinjection was made and when split injection was made;

FIG. 13 is a diagram showing variations in Pi fluctuation rate and otherparameters observed when the proportion of fuel to be injected by laterinjection is varied in various ways; and

FIG. 14 is a diagram showing variations in Pi fluctuation rate and otherparameter observed when later injection start timing is varied invarious ways.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a practical example of a direct injection engine. In thisFigure, designated by the numeral 1 is a main engine body which includesa cylinder block 2 and a cylinder head 3 in which a plurality ofcylinders are formed. A piston 4 is fitted in each cylinder and acombustion chamber 5 is formed between the top surface of the piston 4and the bottom surface of the cylinder head 3.

Their construction is now explained in a specific manner. A recesshaving a particular shape is formed in the bottom surface of thecylinder head 3, the recess forming an upper interior surface of thecombustion chamber 5. For example, the upper interior surface of thecombustion chamber 5 is formed into a pent-roof shape as illustrated,and intake ports 6 and exhaust ports 7 opening into the combustionchamber 5 are formed in its upper interior surface. Although one eachintake port 6 and exhaust port 7 are shown in the Figure, two eachintake ports 6 and exhaust ports 7, individually arranged in a directionperpendicular to the page of the Figure, are provided in a preferredform. Then, an intake valve 8 and an exhaust valve 9 are provided ineach intake port 6 and exhaust port 7, respectively. Driven by anunillustrated valve actuator, the intake valves 8 and the exhaust valves9 are caused to open and close with specified timing.

Spark plugs 10 are fitted in the cylinder head 3 in such a way that eachspark plug 10 is located approximately at the middle of the combustionchamber 5 with its spark gap thrust into the combustion chamber 5.

An injector 11 which injects fuel directly into the combustion chamber 5is provided at a peripheral part of the combustion chamber 5. In theembodiment shown in FIG. 1, the injector 11 is attached to the cylinderhead 3 at the side of the combustion chamber 5 near the intake port 6,and the injector 11 is disposed such that it injects the fuel obliquelydownward with the far end of the injector 11 thrust into the combustionchamber 5.

Also in the illustrated embodiment, a cavity 12 having a U-shaped crosssection is formed in the top of the piston 4 which constitutes thebottom side of the combustion chamber 5. The location and direction ofthe injector 11, the location of the cavity 12 and the location of thespark plug 10 is predetermined to satisfy a particular relationship insuch a way that the fuel is injected from the aforementioned injector 11toward the cavity 12 in the latter half of each compression stroke wherethe piston 4 approaches its top dead center, redirected by the cavity 12and eventually reaches the vicinity of the spark plug 10.

A high-pressure pump 13 is connected to the aforementioned injector 11through a fuel-feeding passage 14. The high-pressure pump 13 and ahigh-pressure regulator which is disposed in an unillustrated returnpassage jointly adjust fuel pressure exerted on the injector 11 to sucha high pressure level that is sufficient to enable fuel injection in amiddle portion of the compression stroke or later.

An intake passage 15 and an exhaust passage 16 are connected to theaforementioned main engine body 1. The aforementioned intake passage 15branches out downstream of a surge tank 15 b to the individualcylinders, whereby two branched passages 15 a (of which only one isshown in the Figure) running parallel to each other are formed for eachcylinder. The two intake ports 6 at the downstream ends of the branchedpassages 15 a open into the combustion chamber 5, and a swirl controlvalve 17 serving as turbulence enhancing means is provided in one ofthese branched passages 15 a. When the swirl control valve 17 is closed,a swirl is produced in the combustion chamber 5 by intake air inductedthrough the other branched passage 15 a so that the turbulence withinthe combustion chamber 5 is enhanced.

As an alternative turbulence enhancing means, a valve which produces atumble may be provided in one branched passage instead of the swirlcontrol valve 17, or there may be made such an arrangement that a squishis produced between the top surface of the piston and the upper interiorsurface of the combustion chamber (bottom surface of the cylinder head)near the top dead center in the compression stroke.

Further, a throttle valve 18 is provided halfway in the intake passage15 and this throttle valve 18 is made controllable by an electricallydriven actuator 19 like a stepper motor to permit control of the amountof intake air.

An exhaust gas recirculation (EGR) passage (not shown) is connected tothe surge tank 15 b via an EGR valve (not shown) to make it possible tointroduce EGR gas upon completion of engine warm-up.

On the other hand, a carbon dioxide (O₂) sensor 21 and a catalyticconverter 22 containing catalysts for converting exhaust gases areprovided in the exhaust passage 16. The above-mentioned O₂ sensor 21detects the air-fuel ratio of a mixture in the combustion chamber bymeasuring the concentration of oxygen and is made of a sensor (λO₂sensor) whose output varies at the stoichiometric air-fuel ratio.

While the catalytic converter 22 may be formed of a three-way catalyst,it is desirable to use catalysts capable of reducing NOx contained evenin a lean mixture having a higher air-fuel ratio than the stoichiometricair-fuel ratio in order to provide increased conversion efficiency whenperforming stratified charge combustion of a lean mixture whose air-fuelratio has been increased after light-off operation, as will be describedlater. More specifically, while the three-way catalyst exhibits highconversion efficiency against all three pollutants HC, Co and NOx onlyin the vicinity of the stoichiometric air-fuel ratio as is commonlyknown, there exists a catalyst (lean NOx catalyst) which not onlyperforms the same function as the three-way catalyst but also reducesNOx even in a lean mixture having a higher air-fuel ratio than thestoichiometric air-fuel ratio. Thus, it is preferable to reduce NOx byusing this catalyst under lean operating conditions. It is to be noted,however, that this kind of lean NOx catalyst also exhibits its maximumconversion efficiency in the vicinity of the stoichiometric air-fuelratio.

Since the catalytic converter 22 contains the lean NOx catalyst, thecatalyst temperature is likely to increase excessively under high-speed,high-load conditions if the catalytic converter 22 is providedimmediately downstream of an exhaust manifold 16 a (or directlyconnected to it). Therefore, the catalytic converter 22 is connecteddirectly to an exhaust pipe 16 b which is connected to the exhaustmanifold 16 a so that the catalytic converter 22 is situated fartheraway from the main engine body 1 than the position immediatelydownstream of the exhaust manifold 16 a. In a case where the three-waycatalyst is used, however, the catalytic converter 22 may be connecteddirectly to the exhaust manifold because the three-way catalyst hasheat-resistant properties.

Designated by the numeral 30 is an ECU (electronic control unit) whichperforms engine control. Signals from a crank angle sensor 23 whichdetects the crank angle of the engine, an acceleration sensor 24 whichdetects accelerator opening (i.e., the amount of operation of anaccelerator pedal), an airflow meter 25 which detects the amount ofintake air, a water temperature sensor 26 which detects the temperatureof engine cooling water, the aforementioned O₂ sensor 21, etc. are inputto the ECU 30.

The aforementioned ECU 30 includes a temperature state identifier 31, aload condition detector 32, a fuel injection controller 33, a fuelinjection amount calculator 34, an ignition timing controller 35 and anengine speed controller 36.

The aforementioned temperature state identifier 31 is for estimating thetemperature state of the catalyst and judging whether it is in anunheated state, in which its temperature is lower than its activationtemperature, based on a temperature sensing signal fed from theaforementioned water temperature sensor 26. If the water temperature islower than a first set temperature, the temperature state identifier 31judges that the catalyst is in its unheated state, and if the watertemperature is higher than the first set temperature, the temperaturestate identifier 31 judges that the catalyst is in its heated state.This temperature state judgment operation for determining catalystlight-off condition may be done by performing water temperaturedetection and a judgment on elapsed time from the point of enginestartup at the same time, or by directly sensing the catalysttemperature.

The aforementioned load condition detector 32 senses the load conditionbased on a crank angle sensing signal fed from the crank angle sensor23.

The aforementioned fuel injection controller 33 is for controlling fuelinjection timing and the amount of fuel to be injected from the injector11 through an injector driving circuit 37. When the catalyst is in itsunheated state, the fuel injection controller 33 controls the injector11 to make split injection including at least two injection cyclesduring a period from an intake stroke to an ignition point at least in alow-load operating range of the engine, the two injection cyclesincluding later injection performed in the middle portion of thecompression stroke or later and earlier injection performed prior to thelater injection.

Here, the middle portion of the compression stroke refers to anintermediate part of the compression stroke as it is divided in threeequal portions, that is, the earlier, middle and later portions.Accordingly, the middle portion of the compression stroke means theperiod from the point of 120° before the top dead center (BTDC) to thepoint of 60° BTDC in terms of the crank angle. Thus, the later injectioncycle is caused to occur at the point of 120° BTDC or later. It ishowever desirable to begin the later injection cycle beforethree-fourths of the period of the compression stroke elapses (not laterthan 45° BTDC), because combustion stability is impaired if the laterinjection timing is too late as will be later discussed.

More specifically, the later injection cycle is set to be commencedwithin a period from 120° ahead of the top dead center to 45° ahead ofthe top dead center during the compression stroke and the earlierinjection cycle is set to be commenced at an appropriate point in timeprior to the later injection cycle, e.g., during the period of theintake stroke, as shown in FIG. 2.

In such split injection performed when the catalyst is in its unheatedstate, the amount of fuel to be injected is calculated by the fuelinjection amount calculator 34 such that the air-fuel ratio falls withina set air-fuel ratio range of 13 to 17, and the amount of fuel to beinjected is divided with specific proportions by the aforementioned fuelinjection controller 33, whereby the amounts of fuel injected by earlierinjection and later injection are controlled.

In the above operation, either of the aforementioned earlier injectionand later injection cycles is so controlled as to inject fuel which willcontribute to main combustion which occurs within a main combustionperiod. Generally, in a combustion process occurring within a combustionchamber, a period in which up to about 10% of the injected fuel in termsof mass is burnt is called an initial combustion period and a period inwhich from about 10% to 90% of the injected fuel is burnt is called themain combustion period. As will be explained later again, initialcombustion in which fuel injected by later injection is ignited andburnt is a combustion cycle encompassing the initial combustion periodand an earlier part of the main combustion period. The amounts of fuelinjected in the individual injection cycles are set in such a way thatthe fuel injected by earlier injection creates a mixture having anair-fuel ratio capable of flame propagation by combustion of the fuelinjected by later injection within the combustion chamber so that bothproportions of fuel injected by earlier injection and later injectioncontribute to main combustion, and the lean mixture produced from thefuel injected by earlier injection is slowly burnt.

Specifically, the proportions of the injected fuel are set such that theair-fuel ratio within the combustion chamber obtained by earlierinjection only becomes 85 or less to achieve an air-fuel ratio capableof flame propagation from a flame caused by combustion of the fuelinjected by later injection and, thus, the proportion of fuel injectedby earlier injection is made equal to ⅕ or more (i.e., the proportion offuel injected by later injection is ⅘ or less) of the total amount ofthe injected fuel. For example, if the air-fuel ratio within the entirecombustion chamber is 17 and the air-fuel ratio within the combustionchamber to be achieved by earlier injection only is 85, the proportionof fuel to be injected by earlier injection is ⅕.

Also, the proportion of fuel injected by later injection is made equalto ⅕ or more (i.e., the proportion of fuel injected by earlier injectionis ⅘ or less) of the total amount of the injected fuel. Thus, theproportion of fuel injected by earlier injection falls within a range of⅕ to ⅘. Preferably, the amount of fuel injected by earlier injection isset such that the air-fuel ratio within the combustion chamber obtainedby earlier injection alone becomes equal to or higher than theflammability limit air-fuel ratio (i.e., a limit of air-fuel ratio atwhich a mixture can be burnt by itself: approximately 30) within theaforementioned range.

It is preferable that the air-fuel ratio within the entire combustionchamber be set, within the aforementioned range of 13 to 17, to a levelequal to or slightly higher than the stoichiometric air-fuel ratio. Theearlier-mentioned injection amount calculator calculates the amount offuel to be injected in such a way that the air-fuel ratio within theentire combustion chamber will match the set air-fuel ratio by way ofopen-loop control or feedback control based on the output from the O₂sensor, as will be described later in greater detail.

Further, the aforementioned ignition timing controller 35 outputs acontrol signal to an ignition device 38 and thereby controls theignition timing according to operating conditions of the engine.Although the ignition timing is essentially controlled to achieve aminimum spark advance for best torque (MBT), it is retarded by aspecified amount when the catalyst is in its unheated state.

The engine speed controller 36 controls the amount of intake air or theignition point, for instance, in such a way that the engine idling speedbecomes higher when the catalyst is in its unheated state than after thecatalyst has been heated.

The aforementioned ECU 30 is so constructed as to control the amount ofintake air as well by outputting a control signal to the actuator 19 fordriving the throttle valve 18. Specifically, the ECU 30 controls theopening of the throttle valve 18 according to the accelerator openingwhen the engine is operated at the stoichiometric air-fuel ratio in ahigh-load range, for instance, when the catalyst is in its unheatedstate or has already been heated, whereas the ECU 30 controls thethrottle valve 18 to open to increase the amount of intake air andthereby increase the air-fuel ratio when stratified charge combustion ismade by injecting the fuel solely in the compression stroke in alow-load range, for instance, after light-off. Further, the ECU 30controls the aforementioned swirl control valve 17 to produce a swirlwithin the combustion chamber 5 in the case of split injection, forinstance.

An example of control operation of this direct injection engine is nowdescribed referring to a time chart of FIG. 3.

In FIG. 3, t₁ designates the point in time of the end of engine startupoperation. Up to the time t₁ during the engine startup operation, theair-fuel ratio is held at a lower level (richer mixture) than thestoichiometric air-fuel ratio to maintain startup torque, for instance,and fuel injection from the injector 11 is made in the intake strokeonly (intake stroke injection). The reason for this arrangement is thatfuel injection in the compression stroke (compression stroke injection),if performed during the engine startup operation, is likely to causepoor evaporation and atomization and eventual misfire due to wetting ofthe spark plug by the fuel and, therefore, it is desirable to makeintake stroke injection to allow for time for fuel evaporation andatomization. In addition, the ignition timing is adjusted to the pointof MBT.

Split injection is made when the catalyst remains in its unheated statebeyond the engine startup end point t₁. More particularly, fuelinjection from the injector 11 is made in two separate parts, that is,earlier injection performed during the intake stroke and later injectionperformed in the middle portion of the compression stroke or later. Inthe example shown in FIG. 3, equal proportions of fuel are allocated tothe earlier injection and later injection cycles.

In this split injection mode, the air-fuel ratio within the entirecombustion chamber is set to fall within a range of 13 to 17. In theexample shown in FIG. 3, the air-fuel ratio is set to a higher level(leaner mixture) than the stoichiometric air-fuel ratio during aspecified time period (t₁ to t₃) immediately following engine startup,where the catalyst temperature is particularly low within the totalperiod when the catalyst is in its unheated state. While the amount offuel to be injected is calculated according to the amount of intake airby open-loop control until the O₂ sensor 21 is activated, the amount offuel to be injected is calculated by feedback control based on theoutput from the O₂ sensor 21 from time t₂ aiming at a “lean” air-fuelratio when the catalyst O₂ sensor 21 has been activated. Further, fromtime t₃ when a specified time period has elapsed, feedback control basedon the output from the O₂ sensor 21 is made aiming at the stoichiometricair-fuel ratio (λ=1).

Furthermore, the ignition timing is retarded while the catalyst is inits unheated state. While the ignition timing is advanced toward the MBTside (or retardation is canceled) and the aforementioned split injectionmode is canceled when the catalyst temperature has increased, theseswitching operations are made with some time lag to alleviate torqueshocks. Especially because ignition timing retardation is ratherdisadvantageous than the aforementioned split injection mode in terms offuel economy (see later-discussed FIG. 10), the ignition timing isadvanced at time t₄ when the catalyst temperature has increased to acertain extent and, then, the split injection mode is canceled at timet₅ when the catalyst has been heated.

It is to be noted that FIG. 3 shows a practical example of controloperation of a stratified charge combustion engine, in which the fuelinjection mode and air-fuel ratio are controlled in accordance withoperating conditions, wherein if the engine is in a low-speed, low-loadrange, for instance, it is switched to compression stroke injection tomake stratified charge combustion and the air-fuel ratio is increased(leaner mixture). Also, if the engine is in a high-speed range or ahigh-load range, it is switched to intake stroke injection to produceuniform combustion. Furthermore, in a region between a stratified chargecombustion region and a uniform combustion region in a medium-loadrange, there are cases where split injection is made encompassing theintake and compression strokes as required to prevent a sudden change intorque.

The engine speed is controlled to match an engine idling speed which isset in accordance with engine cooling water temperature, for instance,upon engine startup. Conventionally, the engine speed is quickly reducedto an ordinary engine idling speed corresponding to the engine coolingwater temperature immediately after engine startup as shown by brokenlines. In this embodiment, however, the engine speed is controlled suchthat it becomes higher than the ordinary engine idling speedcorresponding to the engine cooling water temperature by correcting atarget engine speed, for instance, immediately after engine startup whenthe engine is not heated yet and, then, the engine speed is caused togradually decrease down to the ordinary engine idling speed. Thecombustion stability is enhanced and an ignition timing retarding limitis increased as the engine speed is increased immediately after enginestartup in the above-described manner.

Of fuel control operations among the control operations shown in theaforementioned time chart, the feedback control performed under leanconditions during a time period t₂ to t₃ and the feedback controlperformed at the time t₃ or later are explained with reference to FIGS.4 and 5.

The output of the aforementioned O₂ sensor 21 suddenly changes at thestoichiometric air-fuel ratio (λ=1) as shown in FIG. 4. Generally in thefeedback control based on the output of the O₂ sensor 21, a feedbackcorrection coefficient applied to the amount of injected fuel is madevariable by a P value which is a constant of proportionality and an Ivalue which is an integration constant as shown in FIG. 5, wherein onlyone constant, the P value or I value, is varied in a direction ofdecreasing the amount of injected fuel when the output of the O₂ sensor21 indicates a rich state, whereas only one constant, the P value or Ivalue, is varied in a direction of increasing the amount of injectedfuel when the output of the O₂ sensor 21 indicates a lean state. Inaddition, delay times T_(RL) and T_(LR) are set to allow inversion ofthe feedback correction coefficient when the output of the O₂ sensor 21is inverted from the rich state to the lean state, and from the leanstate to the rich state, respectively.

When fuel injection is to be controlled to obtain a specific air-fuelratio to the “lean” side of the stoichiometric air-fuel ratio during thetime period t₂ to t₃ in the aforementioned feedback control, the delaytime T_(RL) is so adjusted that it becomes larger than the delay timeT_(LR), causing the average value of the feedback correction coefficientto be shifted to the direction of decreasing the amount of injectedfuel. As a consequence, the air-fuel ratio is adjusted in such a waythat it is shifted to the “lean” side of the stoichiometric air-fuelratio. Similar adjustment is possible also by differentiating theaforementioned P or I value depending on whether the output of the O₂sensor 21 indicates the rich state or the lean state.

In the feedback control operation from the time t₃, the delay timesT_(RL) and T_(LR) are adjusted to the same settings on the “rich” sideand the “lean” side to thereby carry out ordinary control operation inwhich the air-fuel ratio is adjusted to the stoichiometric air-fuelratio.

The engine depicted in FIG. 1 is constructed such that with theprovision of the stratified charge-forming cavity 12 in the top of thepiston 4 for capturing the fuel injected from the injector 11 andredirecting it toward the spark plug 10, a stratified charge state inwhich a relatively rich mixture is locally distributed in the vicinityof the spark plug 10 when fuel injection from the injector 11 is made inthe middle portion of the compression stroke or later. The device ofthis invention, however, is applicable not only to the aforementionedtype of engine (hereinafter referred to as the stratified charge engine)but also to a direct injection engine which does not necessarilystratify the mixture (hereinafter referred to as the non-stratifiedcharge engine) with the provision of a piston 41, 42, 43 shown in FIGS.6(a)-6(c), for example.

In this Description, a “flat piston” refers to a piston which is notprovided with the aforementioned cavity 12 for stratified chargeformation. Thus, flat pistons are not limited to those having acompletely flat top like the one shown in FIG. 6(a) but include thosehaving a recessed or protruding top surface to obtain a combustionchamber shape to meet requirements as shown in FIGS. 6(b)-6(c) as longas such recessed or protruding top shape is not intended for stratifiedcharge formation.

Even when the device of the invention is applied to the aforementionednon-stratified charge engine, control of the later injection in whichfuel injection from the injector is made in the middle portion of thecompression stroke or later and the earlier injection performed prior tothe later injection (during an intake stroke period, for example) may bedone as shown in FIG. 3. After the catalyst has been heated, fuelinjection may be made during the intake stroke to produce uniformcombustion. In this case, although fuel injection may be controlled toproduce uniform combustion at λ=1 in all operating ranges, uniformcombustion at a “lean” air-fuel ratio may be made in the low-speed,low-load range.

Operational features of the above-described direct injection engine ofthe present embodiment are described in the following.

If the catalyst is in its unheated state upon engine startup, fuelinjection from the injector is made in two separate parts, that is,earlier injection performed during the intake stroke and later injectionperformed in the middle portion of the compression stroke or later whilecontrolling the amount of fuel to be injected so as to produce anair-fuel ratio approximately equal to or slightly higher than thestoichiometric air-fuel ratio within the entire combustion chamber atleast in the low-load range.

The fuel injected by the earlier injection spreads throughout the entirecombustion chamber and creates a mixture layer which is lean but capableof flame propagation, because a sufficient time is available forevaporation, atomization and spreading prior to ignition. The fuelinjected by the later injection causes a mixture having a relatively lowair-fuel ratio to exist at least in the vicinity of the spark plug 10.Especially in the stratified charge engine shown in FIG. 1, the fuelinjected by the later injection is caused to gather at a highconcentration around the spark plug and, as a consequence, a stratifiedcharge state in which a mixture layer having an air-fuel ratio equal toor higher than the stoichiometric air-fuel ratio is formed is obtained.

As such fuel supply conditions are created, ignition and combustion ofthe mixture are made in a desirable fashion, HC and NOx in the exhaustgases released from the engine are reduced, resulting in an improvementin emissions while the catalyst is in its unheated state, and theexhaust gas temperature increases so that catalyst quick light-offoperation is promoted. These advantageous effects are specificallyexplained referring to FIGS. 7 to 14. Split injection referred to inFIGS. 7 to 14 means split injection in which earlier injection is madeduring the intake stroke and later injection is made during thecompression stroke as shown in the foregoing embodiment.

FIG. 7 shows data on variations in the proportion by mass of combustedfuel after ignition examined when split injection was made and whenintake stroke one-time injection (comparative example) was made underthe following operating conditions:

Engine speed: 1500 rpm

Brake mean effective pressure (Pe): 294 kPa

Ignition timing: Retarded to top dead center (TDC) on compression stroke(in which MBT is about 10° BTDC.)

As shown in this Figure, burning in a latter part of a combustion periodis slower in the split injection than in the aforementioned comparativeexample. This means that the split injection has greater effects inincreasing the exhaust gas temperature by afterburning. Sinceafterburning is sufficiently done in this manner, the catalyst quicklight-off operation is promoted and HC is reduced. Furthermore, as willbe shown in later-described experimental data, NOx is reduced as well.Reasons why such phenomenon occurs are supposed to be as follows.

When later injection is made in the middle portion of the compressionstroke or later, a mixture mass having an air-fuel ratio of λ 1 existsat least locally in the vicinity of the spark plug 10. For example,since a rich mixture layer locally exists around the spark plug 10 inthe stratified charge engine shown in FIG. 1, the ignition stability isensured and combustion after ignition is properly made, so that theburning velocity in initial combustion is increased.

Further, the fuel injected by earlier injection spreads throughout theentire combustion chamber and creates a lean mixture, and as burning ofthe mixture produced by the earlier-described later injection proceeds,a flame propagates to the lean mixture which was produced by the fuelinjected by the earlier injection and mixed with part of the fuelinjected by the later injection, whereby the lean mixture mass is burnt.To summarize, burning of the mixture produced by the later injection andsucceeding burning of the lean mixture produced mainly by the fuelinjected by the earlier injection are made during the main combustionperiod. Since the burning of the lean mixture is a slow combustionprocess, it serves to suppress generation of NOx.

It is thus supposed that the catalyst quick light-off operation ispromoted with an increase in the exhaust gas temperature and HC isoxidized and reduced.

The aforementioned phenomenon is also accomplished in a non-stratifiedcharge engine employing flat pistons shown in FIG. 6. Specifically, thetime period from later injection in the middle portion of thecompression stroke or later to ignition is so short that the fuel is notdispersed in a completely uniform fashion, if not stratified, and acondition in which relatively rich mixture masses and lean mixturemasses are randomly scattered is created in the non-stratified chargeengine as well. Since a locally rich mixture exists near the spark plug,ignition and combustion are made in a desirable fashion, and because thefuel injected by earlier injection forms a uniform and lean mixture, aflame propagates toward it and this mixture is slowly burnt.

Referring again to FIG. 7, the proportion by mass of combusted fuel inan earlier part of combustion rises more quickly in the split injectionthan in the intake stroke one-time injection, and this indicates thatthe combustion stability is high. Such phenomenon is conspicuouslyobserved in the non-stratified charge engine employing the flat pistonsshown in FIG. 6, as well as in the stratified charge engine shown inFIG. 1 when the amount of fuel injected in the split injection mode issmall and the degree of stratification is relatively low, for example.Reasons why such phenomenon occurs are supposed to be as follows.

Since the condition in which relatively rich mixture masses and leanmixture masses are randomly scattered is created by the later injectionin the non-stratified charge engine as described above, the flamepropagation velocity becomes uneven and irregular recesses andprotrusions are formed in a flame front in the process of flamepropagation. It is supposed that burning in the earlier part ofcombustion is promoted because the aforementioned unevenness increasesthe surface area of the flame and contributes to promotion ofcombustion.

Also in the stratified charge engine, the time period from the laterinjection to ignition is so short that the mixture produced by the fuelinjected by the later injection is locally distributed around the sparkplug, and because there is unevenness in the distribution of theair-fuel ratio even within the locally distributed region, there existrelatively rich mixture masses and relatively lean mixture masses aroundthe spark plug when the degree of stratification is relatively low. Itis supposed that the surface area of the flame burning in the earlierpart of combustion is promoted because the flame propagation velocitybecomes uneven and this increases the surface area of the flame.

As it becomes possible to increase the amount of ignition timingretardation when the combustion stability is enhanced as describedabove, it is possible to further increase the exhaust gas temperature byretardation of the ignition timing and thereby enhance the effects ofquick light-off and reduction of HC and other emissions in addition tothe earlier-mentioned effects of increasing the exhaust gas temperatureby afterburning.

FIG. 8 shows data on variations in the surface area of a flame (or flamefront area) in relation to the crank angle from the ignition pointobserved when intake stroke one-time injection and split injection weremade to examine the effects of split injection on the flame front area.As can be seen from this Figure, the flame front area increases rapidlyand the combustion stability is enhanced by split injection compared toone-time injection.

FIG. 9 shows variations in fuel economy rate and exhaust gas temperatureobserved when the later injection timing is altered with the ignitiontiming varied from the point of MBT to a retarding side in intake strokeone-time injection and with the ignition timing set to the point of MBTin split injection. Operating conditions used were an engine speed of1500 rpm and a brake mean effective pressure (Pe) of 294 kPa. As can beseen from this Figure, the exhaust gas temperature increases and thefuel economy rate deteriorates as the injection timing is progressivelyretarded in the case of intake stroke one-time injection. On the otherhand, the exhaust gas temperature increases and the fuel economy ratedeteriorates as the later injection timing is progressively retardedfrom about 90° BTDC (before the top dead center) on the compressionstroke in the case of split injection.

A comparison of these cases indicates that the fuel economy rate isdecreased in split injection when the exhaust gas temperature is thesame. (The exhaust gas temperature increases by 60° C. when theinjection timing is retarded from the point of MBT in intake strokeone-time injection, for example.) To summarize, the exhaust gastemperature can be more increased by retarding the ignition timing insplit injection than in one-time injection provided that thedeterioration in fuel economy is kept approximately to the same level.Furthermore, it will be possible to increase the exhaust gas temperatureto an even greater extent if the ignition timing is retarded whileperforming the split injection.

FIG. 10 shows measurement results of exhaust gas temperature, HC and NOxconcentrations in exhaust gases released from the combustion chamber andengine speed fluctuation coefficient ΔRPM (standard deviation) takenfrom a testing of a comparative example in which the ignition timing wasretarded in intake stroke one-time injection and of the practicalexample of the invention in which the ignition timing was retarded insplit injection, the testing being conducted with the amount of ignitiontiming retardation adjusted to equalize the amount of fuel consumptionfor the two examples (the ignition timing was retarded up to TDC in boththe comparative example and the practical example) at an engine speed of1500 rpm under low-load operating conditions. As can be seen from thisFigure, the exhaust gas temperature is significantly more increased, theHC and NOx concentrations are more reduced and the engine speedfluctuation coefficient ΔRPM is more reduced in the practical example ofthis invention than in the comparative example even under the sameoperating conditions and at the same amount of fuel consumption.

Reasons for this are supposed to be that the exhaust gas temperature isincreased and HC is reduced since burning in the latter part ofcombustion is slowed down by split injection as described earlier, NOxis reduced because burning of a lean mixture produced by earlierinjection becomes a slow combustion process, for instance, thecombustion stability is enhanced due to promotion of burning in theearlier part of the combustion period, and so forth.

FIG. 11 shows measurement results of HC reduction rate, NOx reductionrate and exhaust gas temperature taken when one-time injection wasconducted in the intake stroke and when split injection was conductedwhile driving a motor vehicle equipped with the direct injection engine.As can be seen from this Figure, the increase in the exhaust gastemperature is accelerated in the case of split injection compared tothe case of intake stroke one-time injection and, as a consequence,periods of time individually required for the HC reduction rate and NOxreduction rate reach 50% are significantly reduced (by ta and tb asillustrated, respectively).

FIG. 12 shows data on the relationship between the ignition timing andindicated mean effective pressure examined when intake stroke one-timeinjection was made and when split injection was made. As can be seenfrom this Figure, although the indicated mean effective pressure(torque) decreased when the ignition timing is retarded, the degree ofreduction in the indicated mean effective pressure is smaller in splitinjection than in intake stroke one-time injection.

It is recognized from the aforementioned data that HC and NOx in theexhaust gases released from the engine are reduced, resulting in animprovement in emissions, and the catalyst light-off operation ispromoted as a result of an increase in the exhaust gas temperature bymaking split injection when the catalyst is in its unheated state as inthe present invention. Moreover, the combustion stability and fueleconomy are improved in this invention compared to the case in which theignition timing is retarded by a large amount in one-time injection.

Referring to FIGS. 13 and 14, desirable ranges of the proportion of fuelto be injected by the later injection and earlier injection and of thetiming of the later injection are explained in the following.

FIG. 13 shows data on the relationship among the proportion of fuel tobe injected by the later injection (or the ratio of the amount of fuelinjected by the later injection to the amount of fuel injected by theearlier injection) and Pi (indicated mean effective pressure)fluctuation rate, exhaust gas temperature, fuel economy rate, the amountof HC emissions and the amount of NOx emissions. Operating conditionsused were an engine speed of 1500 rpm, a brake mean effective pressure(Pe) of 294 kPa and an engine cooling water temperature of 40.0° C., inwhich the ignition timing was retarded to the top dead center (TDC) oncompression stroke. As can be seen from this Figure, exhaust gastemperature increasing effects and HC and NOx reduction effects are notsufficiently obtained if the proportion of fuel injected by the laterinjection is smaller than 20% (⅕). When the proportion of fuel injectedby the later injection becomes equal to or larger than 20% (⅕), theexhaust gas temperature increasing effects and the HC and NOx reductioneffects increase with an increase in the proportion of fuel injected bythe later injection, but the Pi fluctuation rate and fuel economy rategradually increase. When the proportion of fuel injected by the laterinjection exceeds 80%, the Pi fluctuation rate exceeds its permissiblelevel and the combustion stability is lost.

Accordingly, it is desirable to keep the proportion of fuel injected bythe later injection within a range of 20% to 80% (⅕ to ⅘) in order tomaintain the combustion stability and torque while ensuring the exhaustgas temperature increasing effects and HC and NOx reduction effects.When this is accomplished, the proportion of fuel injected by theearlier injection falls within a range of ⅘ to ⅕. In addition, theexhaust gas temperature increasing effects and HC and NOx reductioneffects become greater as the proportion of fuel injected by the laterinjection is increased, that is, as the proportion of fuel injected bythe earlier injection is decreased, as long as these proportions fallwithin the aforementioned ranges. If the amount of fuel injected by theearlier injection is set to such a low level that the air-fuel ratiowithin the entire combustion chamber produced by only the earlierinjection becomes equal to or higher than the flammability limitair-fuel ratio (approximately 30), the mixture created by the earlierinjection becomes sufficiently lean. Since this lean mixture burnsslowly, retarding the burning in the latter part of the combustionperiod, it is possible to obtain sufficient exhaust gas temperatureincreasing effects and HC and NOx reduction effects.

Further, if the amount of fuel injected by the later injection is madesmaller than the amount of fuel injected by the earlier injection (lessthan 50% in terms of the proportion of fuel injected by the laterinjection), the combustion stability is increased and fuel consumptionis reduced. On the other hand, if the amount of fuel injected by thelater injection is made larger than the amount of fuel injected by theearlier injection (greater than 50% in terms of the proportion of fuelinjected by the later injection), the exhaust gas temperature increasingeffects and HC and NOx reduction effects are enhanced.

In an extreme low-load range, like the idling range, in which the amountof fuel supply to the combustion chamber is small, injectionpulselengths corresponding to the divided amounts of injected fuel (orthe pulselengths which determine valve-opening periods of the injector)approach a minimum controllable injection pulselength. Accordingly, ifthe amount of fuel injected by the earlier injection differs from theamount of fuel injected by the later injection, there arises apossibility that the injection pulselength corresponding to the smalleramount of fuel to be injected becomes smaller than the minimum injectionpulselength, making it difficult to control the amount of fuel to beinjected. In such circumstances, it is desirable to equalize the amountof fuel injected by the later injection and earlier injection (50% offuel injected by the later injection).

FIG. 14 shows the relationship between later injection start timing andthe Pi fluctuation rate and exhaust gas temperature examined underoperating conditions of an engine speed of 1500 rpm, a brake meaneffective pressure (Pe) of 294 kPa and an engine cooling watertemperature of 40.0° C., in which the ignition timing was retarded tothe top dead center (TDC) on compression stroke. As can be seen fromthis Figure, the exhaust gas temperature increasing effects are scarcelyobtained when the later injection starting point precedes 120° BTDC. Theexhaust gas temperature increasing effects are enhanced when the laterinjection starting point is retarded to or beyond 120° BTDC. When the Pifluctuation rate increases and the later injection starting point isretarded beyond 60° BTDC, however, the Pi fluctuation rate exceeds itspermissible level and the combustion stability is lost.

If the later injection starting point is set within a range of 120° BTDCto 60° BTDC when the ignition point is retarded to TDC, it is possibleto provide appropriate fuel evaporation and atomization times and, as aconsequence, the exhaust gas temperature increasing effects are obtainedwhile securing the combustion stability. As it is possible to enhancethe exhaust gas temperature increasing effects if the later injectionstarting point is not retarded up to TDC, the later injection startingpoint may be set within a range of 120° BTDC to 45° BTDC.

In the extreme low-load range in which the amount of fuel supply to thecombustion chamber is small, it is possible to sufficiently retard theinjection timing while keeping the later injection starting point in arange not later than 45° BTDC from the viewpoint of fuel evaporation andatomization.

Moreover, if the amount of fuel to be injected by the later injection isrelatively small, the later injection starting point may be set withinthe range of 120° BTDC to 45° BTDC while ensuring that the injectionpoint lies beyond TDC.

According to the construction shown in FIG. 1 and the control operationshown in the time chart of FIG. 3, it is possible to obtain furtheroperational features and effects which are described below.

As the swirl control valve 17 shown in FIG. 1 is closed at least in thelow-load range when the catalyst is not heated yet, it produces a swirlwithin the combustion chamber 5 and thereby enhances turbulence withinthe combustion chamber 5. As such turbulence enhancing means like theswirl control valve 17 is provided, the combustion stability isincreased by enhancement of turbulence within the combustion chamberwhen the aforementioned split injection is conducted at least in thelow-load range while the catalyst is still in its unheated state. It istherefore possible to maintain the combustion stability whilesuppressing increase in the Pi fluctuation rate even when the laterinjection starting point is retarded by a relatively large amount toincrease the quick light-off effects in split injection, and thecatalyst quick light-off operation is promoted even further because theignition timing retarding limit is increased.

In the control operation shown in the time chart of FIG. 3, the air-fuelratio is set to a “lean” level during the specified time period (t₁ tot₃) in which the catalyst temperature is particularly low within thetotal period when the catalyst is in its unheated state, so that HC andother emissions in the exhaust gases are reduced. Furthermore, thereexists excess oxygen when the air-fuel ratio is set to the “lean” levelas described above and, and this makes it possible to perform sufficientafterburning of the fuel injected by split injection and is advantageousfor quick light-off. From the specified point in time t₃ when thecatalyst temperature has increased to a certain extent and the catalystbecomes more or less activated, though it is still in its unheatedstate, the air-fuel ratio is set to the stoichiometric air-fuel ratio(λ=1). Consequently, HC and NOx are reduced by the conversion effects ofthe catalyst and its reaction also serves to promote the catalyst quicklight-off operation.

The manner of air-fuel ratio control operation is not limited to theforegoing practical example. For example, the air-fuel ratio may be setto a “leaner” level than the stoichiometric air-fuel ratio (but notexceeding 17) during a particular period when the catalyst in itsunheated state, or the air-fuel ratio may be controlled such that itmatches the stoichiometric air-fuel ratio from a point in timeimmediately after engine startup.

Also in FIG. 3, split injection for quick light-off is conducted and theignition timing is retarded when the catalyst is still in its unheatedstate, and to aid in reducing torque shocks occurring when cancelingsuch control operation and in improving fuel economy, the ignitiontiming is advanced toward the MBT side when the catalyst temperature hasincreased and, then, the aforementioned split injection mode iscanceled. This process may, however, be altered such that theaforementioned advancement of the ignition timing and cancellation ofthe split injection mode are made at the same time.

The timing of earlier injection in the split injection mode when thecatalyst is in its unheated state is not limited to the intake strokeperiod but may be at any point during the intake stroke or later as longas that point exists prior to the later injection. As an example, theearlier injection may be made during the earlier portion of thecompression stroke.

The invention thus far described by way of specific example provides thefollowing features and advantages.

In one aspect of the invention, a control device for a direct injectionengine having a catalyst provided in an exhaust passage for convertingexhaust gases and an injector for injecting fuel directly into acombustion chamber comprises a temperature state identifier for judgingthe temperature state of the catalyst, and a fuel injection controllerfor controlling fuel injection from the injector, wherein the fuelinjection controller controls the injector based on judgment results ofthe temperature state identifier in such a way that the injector makesat least two-step split injection during a period from an intake stroketo an ignition point including a later injection cycle performed in amiddle portion of a compression stroke or later and an earlier injectioncycle performed prior to the later injection cycle at least in alow-load range of the engine when the catalyst is in its unheated state,in which its temperature is lower than its activation temperature, andeither of the later injection cycle and earlier injection cycle injectsfuel which contributes to main combustion during a main combustionperiod in which approximately 10% to 90% by mass of the injected fuel isburnt in a combustion process occurring in the combustion chamber.

According to this aspect of the invention, the injector makes splitinjection at least in the low-load range of the engine when the catalystis in its unheated state. The later injection cycle performed in themiddle portion of the compression stroke or later produces unevenness ina mixture in which locally rich mixture masses are created. Sincerelatively rich mixture masses are scattered or locally distributed neara spark plug, ignitability and combustion stability after ignition areensured, and because a uniform and lean mixture layer is formed by theearlier injection cycle, the combustion slows down in a latter part ofthe combustion period and continues until a relatively later time. Dueto such slow burning in the latter part of the combustion period (whichis known as afterburning), HC and NOx in the exhaust gases released fromthe combustion chamber are reduced, resulting in an improvement inemissions while the catalyst is in its unheated state, and the exhaustgas temperature is increased so that catalyst light-off operation issignificantly promoted.

In another aspect of the invention, a control device for a directinjection engine having a catalyst an exhaust passage and an injectorfor injecting fuel directly into a combustion chamber comprises atemperature state identifier for judging the temperature state of thecatalyst which is provided in the exhaust passage for converting exhaustgases, and a fuel injection controller for controlling fuel injectionfrom the injector, wherein the fuel injection controller controls theinjector based on judgment results of the temperature state identifierin such a way that the injector makes at least two-step split injectionduring a period from an intake stroke to an ignition point including alater injection cycle performed in a middle portion of a compressionstroke or later and an earlier injection cycle performed prior to thelater injection cycle regardless of whether the engine has already beenheated or not but when the catalyst is in its unheated state, in whichits temperature is lower than its activation temperature, and either ofthe later injection cycle and earlier injection cycle injects fuel whichcontributes to main combustion.

According to this aspect of the invention, ignitability and combustionstability while the catalyst is in its unheated state are ensured by thesplit injection regardless of whether the engine has already been heatedor not. Further, exhaust gas temperature increasing effects are obtaineddue to afterburning so that an improvement in emissions while thecatalyst is in its unheated state and the promotion of catalystlight-off operation are accomplished.

In the aforementioned form of the invention, the amount of fuel injectedin the earlier injection cycle of the split injection performed when thecatalyst is in its unheated state is such an amount that produces a leanmixture which has a higher air-fuel ratio than the stoichiometricair-fuel ratio and is capable of flame propagation at least by fuelinjected in the later injection cycle and combustion thereof.

The aforementioned air-fuel ratio capable of flame propagation by thefuel injected in the later injection cycle and combustion thereofproduced in the combustion chamber by the earlier injection cycle aloneshould preferably set to 85 or less.

More specifically, it is preferable that the air-fuel ratio in theentire combustion chamber be set to fall within a range of 13 to 17 whenthe catalyst is in its unheated state, the amount of fuel injected inthe earlier injection cycle of the split injection be set to ⅕ or aboveof the total amount of the injected fuel, and the air-fuel ratio in thecombustion chamber achieved by the earlier injection cycle alone be setto a level equal to or higher than the flammability limit air-fuelratio. Further, it is preferable set the amount of fuel injected in thelater injection cycle of the split injection to ⅕ or above of the totalamount of the injected fuel.

With this arrangement, ignition and subsequent combustion in laterinjection are made satisfactorily and the fuel injected in the earlierinjection cycle is burnt by flame propagation by the fuel injected inthe later injection cycle and combustion thereof. As afterburning ismade in this manner, it is possible to gain such advantageous effects asa reduction in HC, NOx and other emissions, an increase in exhaust gastemperature, and consequent the quick light-off operation.

A reason why the air-fuel ratio is to be set within the range of 13 to17 is that a high heat release rate is obtained in this range ofair-fuel ratio and it is possible to use an air-fuel ratio capable ofincreasing the exhaust gas temperature. Furthermore, it is possibleprevent an increase in NOx emissions in the exhaust gases released fromthe combustion chamber and obtain exhaust gas temperature increasingeffects by setting the amount of fuel injected in the earlier injectioncycle to ⅕ or above of the total amount of the injected fuel. Moreover,combustion stability is ensured by setting the amount of fuel injectedin the later injection cycle to ⅕ or above of the total amount of theinjected fuel (the amount of fuel injected in the earlier injectioncycle to ⅘ or less of the total amount of the injected fuel).

The total amount of the injected fuel referred to above means the totalamount of fuel injected during the period from the intake stroke to theignition point.

In split injection, there is such a relationship between the amount offuel injected in the earlier injection cycle and the amount of fuelinjected in the later injection cycle that the exhaust gas temperatureincreasing effects are increased if the former is made smaller than thelatter.

As an alternative, the amount of fuel injected in the earlier injectioncycle of the split injection performed when the catalyst is in itsunheated state may be made larger than the amount of fuel injected inthe later injection cycle. This alternative approach will serve toincrease the combustion stability.

As another alternative, the amount of fuel injected in the earlierinjection cycle and the amount of fuel injected in the later injectioncycle may be made equal to each other in the split injection performedwhen the catalyst is in its unheated state. Especially in an extremelow-load range, like the idling range, in which the amount of fuelsupply to the combustion chamber is small, the divided amounts of fuelto be injected approach a minimum controllable amount of fuel to beinjected (minimum injection pulselength). Accordingly, it is preferableto make these amounts of fuel to be injected equal to each other ifthere exists a possibility that the smaller amount of fuel to beinjected becomes smaller than the minimum controllable amount of fuel tobe injected when the amount of fuel injected by the earlier injectiondiffers from the amount of fuel injected by the later injection.

In the aforementioned control device of the invention, it is desirableto begin the later injection cycle before three-fourths of the period ofthe compression stroke elapses in the split injection performed when thecatalyst is in its unheated state. This is because if the laterinjection starting point is further delayed, torque fluctuation rateincreases and the combustion stability is lost.

If there is provided an ignition timing controller which retards theignition timing by a specified amount beyond the MBT when theaforementioned split injection is performed while the catalyst is in itsunheated state, it is possible to achieve the exhaust gas temperatureincreasing effects by retarding the ignition timing. In particular,retardation of the ignition timing combined with the split injection,which ensures combustibility, produces synergistic effects of the quicklight-off operation.

When retarding the ignition timing in the aforementioned manner, it isadvantageous if the later injection cycle of the split injectionperformed when the catalyst is in its unheated state is commenced beforethe middle portion of the compression stroke elapses. In other words, ifthe later injection starting point is retarded beyond this point oncondition that the ignition timing is retarded, the torque fluctuationrate increases and the combustion stability is lost.

Also, when the catalyst temperature has increased after starting splitinjection and retardation of the ignition timing when the catalyst is inits unheated state, the split injection mode is canceled and theignition timing is advanced toward the MBT side in this order, or inreverse order. Torque shocks are reduced by performing the operationsfor the cancellation of the split injection mode and advancement of theignition timing toward the MBT side with some time lag in this fashion.

It is however preferable to first perform the operation for advancingthe ignition timing toward the MBT side when the catalyst temperaturehas increased after starting the split injection and retardation of theignition timing while the catalyst is in its unheated state. This isbecause if the aforementioned operation for the cancellation of thesplit injection mode is first performed, a deterioration incombustibility is likely to be caused by the retardation of the ignitiontiming.

In one approach, after starting split injection and retardation of theignition timing when the catalyst is in its unheated state, the ignitiontiming may be advanced toward the MBT side as soon as the splitinjection mode has been canceled.

Since a deterioration in combustibility can be caused by the retardationof the ignition timing in certain cases when the split injection isstopped, the ignition timing is advanced as soon as the split injectionmode has been canceled at the latest.

If the air-fuel ratio within the entire combustion chamber is setapproximately to the stoichiometric air-fuel ratio when the catalyst isin its unheated state in the invention, sufficient exhaust gastemperature increasing effects are obtained, and the catalyst begins toexhibits its conversion effects when it is activated to a certain degreeeven before it reaches a fully heated condition.

Proper control of the air-fuel ratio will be achieved if there areprovided an O₂ sensor whose output varies at the stoichiometric air-fuelratio and an injection amount calculator which calculates the amount offuel to be injected by feedback control such that the air-fuel ratiomatches the stoichiometric air-fuel ratio based on the output of the O₂sensor after it has become activated when the catalyst is in itsunheated state.

The air-fuel ratio within the entire combustion chamber may be set to aleaner level than the stoichiometric air-fuel ratio within a range notexceeding 17 when the catalyst is in its unheated state. If the air-fuelratio is set to a more or less leaner level when the catalyst is in itsunheated state, it will be advantageous in reducing HC and NOx in theexhaust gases released from the engine.

For this purpose, it is preferable that the control device comprise anO₂ sensor whose output varies at the stoichiometric air-fuel ratio, andan injection amount calculator for calculating the amount of fuel to beinjected by feedback control based on the output of the O₂ sensor afterit has become activated when the catalyst is in its unheated state,wherein the injection amount calculator sets the air-fuel ratio at apoint offset to the lean side of the stoichiometric air-fuel ratio byadjusting one of such factors as a delay time of change in a controlvalue applicable when the output of the O₂ sensor is inverted, aconstant of proportionality and an integration constant.

According to this configuration, it is possible to exercise effectivefeedback control even when producing a lean condition when the catalystis in its unheated state by using a λO₂ sensor (a type of O₂ sensorwhose output varies at the stoichiometric air-fuel ratio) so that itwill be advantageous when performing feedback control at thestoichiometric air-fuel ratio after the catalyst has been heated.

In a case where the air-fuel ratio within the entire combustion chamberis controlled to a leaner level than the stoichiometric air-fuel ratiowhen the catalyst is in its unheated state as described above, it willbe advantageous to vary the air-fuel ratio to the “richer” side inaccordance with subsequent increase in catalyst temperature. Morespecifically, if the air-fuel ratio is varied from the lean condition tothe “richer” side, or to match the stoichiometric air-fuel ratio, forexample, when the catalyst temperature has increased to such a levelthat the catalyst begins to be activated, it will be advantageous forusing the conversion effects of the catalyst and the quick light-offeffects will be further increased.

In a further aspect of the invention, a control device for a directinjection engine having a catalyst provided in an exhaust passage forconverting exhaust gases and an injector for injecting fuel directlyinto a combustion chamber comprises a temperature state identifier forjudging the temperature state of the catalyst, and a fuel injectioncontroller for controlling fuel injection from the injector, wherein thefuel injection controller controls the injector based on judgmentresults of the temperature state identifier in such a way that theinjector makes two-step split injection including an earlier injectioncycle performed during the period of an intake stroke and a laterinjection cycle commenced in a middle portion of a compression stroke orlater but not later than 45° before top dead center thereof when thecatalyst is in its unheated state, in which its temperature is lowerthan its activation temperature, and wherein the fuel injectioncontroller sets the air-fuel ratio in the entire combustion chamber tofall within a range of 13 to 17 and the amount of fuel injected in theearlier injection cycle to fall within a range of about ⅕ to about ⅘ ofthe total amount of the injected fuel.

Since the total amount of the injected fuel is so adjusted that theair-fuel ratio in the entire combustion chamber falls within the rangeof 13 to 17 when the catalyst is in its unheated state in this aspect ofthe invention, it is possible to use the air-fuel ratio which provides ahigh heat release rate suited for increasing the exhaust gastemperature. Also, since the proportion of the fuel injected in theearlier injection cycle is set to about ⅕ or above, an increase of NOxin the exhaust gases released from the combustion chamber is avoided andexhaust gas temperature increasing effects are achieved. In addition,combustion stability is ensured since the proportion of the fuelinjected in the earlier injection cycle is controlled to about ⅘ orless. As about ⅕ to ⅘ of the total amount of fuel input is injected inthe earlier injection cycle during the intake stroke and the remainderof the total fuel input is injected in the middle portion of thecompression stroke or later as described above, ignition and subsequentcombustion are made satisfactorily and slow burning is made in thelatter part of the combustion period. As a result, HC, NOx and otheremissions are reduced, and the exhaust gas temperature is increased sothat effects of promoting catalyst quick light-off operation issufficiently exhibited.

In the above-described control device of the invention, the laterinjection cycle of the split injection performed when the catalyst is inits unheated state should preferably be commenced in a period not laterthan 45° before top dead center of the compression stroke. It ispreferable that the later injection cycle of the split injection becommenced in a period from 120° before top dead center of thecompression stroke to 45° before top dead center thereof. Combustionstability is jeopardized if the later injection starting point occurslater than the aforementioned periods, whereas unevenness in mixtureconcentration is not sufficiently produced if the later injectionstarting point occurs earlier than the aforementioned periods.

Preferably, the amount of fuel injected in the earlier injection cycleof the split injection performed when the catalyst is in its unheatedstate should be such an amount that produces a lean mixture which has ahigher air-fuel ratio than the stoichiometric air-fuel ratio and iscapable of flame propagation at least by fuel injected in the laterinjection cycle and combustion thereof, and the lean air-fuel ratiohigher than the stoichiometric air-fuel ratio capable of flamepropagation by the fuel injected in the later injection cycle andcombustion thereof produced in the combustion chamber by the earlierinjection cycle alone should be set to 85 or less.

In an extreme low-load range, like the idling range, in which the amountof fuel supply to the combustion chamber is small, the divided amountsof fuel to be injected approach a minimum controllable amount of fuel tobe injected (minimum injection pulselength). Accordingly, it ispreferable to make these amounts of fuel to be injected equal to eachother if there exists a possibility that the smaller amount of fuel tobe injected becomes smaller than the minimum controllable amount of fuelto be injected when the amount of fuel injected by the earlier injectiondiffers from the amount of fuel injected by the later injection.

When the catalyst has reached its heated state and its temperature hasbecome equal to or higher than its activation temperature, mode of fuelinjection from the injector may be switched to intake stroke injectionor to compression stroke injection as is commonly done in controloperation of this type of engines.

Effects of quick light-off are enhanced if the control device of thisinvention further comprises an ignition timing controller for retardingan ignition point by a specified amount from a minimum spark advance forbest torque (MBT) when the split injection is performed while thecatalyst is in its unheated state. In this case, it is preferable toretard the ignition point by a specified amount from the MBT when thecatalyst is in its unheated state and then advance the ignition pointtoward the MBT according to an increase in catalyst temperature.

According to the invention, the air-fuel ratio in the entire combustionchamber may be set to a level equal to or higher than the stoichiometricair-fuel ratio when the catalyst is in its unheated state. This makes itpossible to obtain an air-fuel ratio advantageous in reducing HC and NOxin the exhaust gases released from the engine and the catalyst quicklight-off operation.

Proper control of the air-fuel ratio advantageous in reducing HC and NOxand the catalyst quick light-off operation will be achieved if there areprovided an O₂ sensor whose output varies at the stoichiometric air-fuelratio and an injection amount calculator which calculates the amount offuel to be injected by feedback control such that the air-fuel ratiomatches the stoichiometric air-fuel ratio based on the output of the O₂sensor after it has become activated when the catalyst is in itsunheated state.

The aforementioned control device may further comprise an O₂ sensorwhose output varies at the stoichiometric air-fuel ratio and aninjection amount calculator for calculating the amount of fuel to beinjected by feedback control based on the output of the O₂ sensor afterit has become activated when the catalyst is in its unheated state,wherein the injection amount calculator sets the air-fuel ratio at apoint offset to the lean side of the stoichiometric air-fuel ratio byadjusting one of such factors as a delay time of change in a controlvalue applicable when the output of the O₂ sensor is inverted, aconstant of proportionality and an integration constant.

The control device may be such that the air-fuel ratio in the entirecombustion chamber is set to a level higher than the stoichiometricair-fuel ratio when the catalyst is in its unheated state and then theair-fuel ratio is varied to the rich side with an increase in catalysttemperature.

The control device may further comprise a turbulence enhancer forenhancing turbulence within the combustion chamber when the catalyst isin its unheated state. In this construction, combustibility is increasedby the enhanced turbulence and the ignition timing retarding limit isincreased when the split injection is performed, so that the catalystlight-off operation is even more promoted.

Furthermore, there may be provided an engine speed controller forcontrolling the engine such that its idling speed becomes higher whenthe catalyst is in its unheated state than after the catalyst has beenheated. As the engine speed is increased while the split injection isperformed, combustibility is increased and the ignition timing retardinglimit is increased, so that the catalyst light-off operation is evenmore promoted.

In a still further aspect of the invention, a control device for adirect injection engine having a catalyst provided in an exhaust passagefor converting exhaust gases and an injector for injecting fuel directlyinto a combustion chamber comprises a temperature state identifier forjudging the temperature state of the catalyst, a load condition detectorfor sensing engine load conditions, and a fuel injection controller forcontrolling fuel injection from the injector, wherein the fuel injectioncontroller controls the injector based on judgment results of thetemperature state identifier and on sensing results of the loadcondition detector in such a way that the injector injects fuel in acompression stroke to have the engine perform stratified chargecombustion in its low-load range and injects fuel in an intake stroke tohave the engine perform uniform combustion in its high-load range whenthe catalyst is in its heated state, in which its temperature is equalto or higher than its activation temperature, and the injector makestwo-step split injection including an earlier injection cycle performedduring the period of the intake stroke and a later injection cyclecommenced in a middle portion of the compression stroke or later but notlater than 45° before top dead center thereof at least in a low-loadrange of the engine when the catalyst is in its unheated state, in whichits temperature is lower than its activation temperature, and whereinthe fuel injection controller sets the air-fuel ratio in the entirecombustion chamber to fall within a range of 13 to 17 and the amount offuel injected In the earlier injection cycle to fall within a range ofabout ⅕ to about ⅘ of the total amount of the injected fuel in the splitinjection.

According to this aspect of the invention, effects of quick light-offand HC and NOx reduction are obtained when the catalyst is in itsunheated state, and stratified charge combustion and uniform combustion,for example, are performed according to opera ting conditions after thecatalyst has reached in its heated state.

In a yet further aspect of the invention, a control device for a directinjection engine having a catalyst provided in an exhaust passage forconverting exhaust gases and an injector for injecting fuel directlyinto a combustion chamber comprises a temperature state identifier forjudging the temperature state of the catalyst, and a fuel injectioncontroller for controlling fuel injection from the injector, wherein thefuel injection controller controls the injector based on judgmentresults of the temperature state identifier in such a way that theinjector injects fuel in an intake stroke to have the engine performuniform combustion when the catalyst is in its heated state, in whichits temperature is equal to or higher than its activation temperature,and the injector makes two-step split injection including an earlierinjection cycle performed during the period of the intake stroke and alater injection cycle commenced in a middle portion of the compressionstroke or later but not later than 45° before top dead center thereof atleast in a low-load range of the engine when the catalyst is in itsunheated state, in which its temperature is lower than its activationtemperature, and wherein the fuel injection controller sets the air-fuelratio in the entire combustion chamber to fall within a range of 13 to17 and the amount of fuel injected in the earlier injection cycle tofall within a range of about ⅕ to about ⅘ of the total amount of theinjected fuel in the split injection.

According to this aspect of the invention, effects of quick light-offand HC and NOx reduction are obtained when the catalyst is in itsunheated state, and uniform combustion is performed after the catalysthas reached in its heated state.

It is preferable that the aforementioned control device of the inventionfurther comprise an ignition timing controller for retarding an ignitionpoint by a specified amount from the MBT.

Furthermore, it is preferable that the air-fuel ratio in the entirecombustion chamber be set to a level equal to or higher than thestoichiometric air-fuel ratio when the catalyst is in its unheatedstate.

Moreover, it is preferable that the aforementioned control device of theinvention further comprise an O₂ sensor whose output varies at thestoichiometric air-fuel ratio, and an injection amount calculator forcalculating the amount of fuel to be injected by feedback control suchthat the air-fuel ratio matches the stoichiometric air-fuel ratio, basedon the output of the O₂ sensor after it has become activated when thecatalyst is in its unheated state.

INDUSTRIAL APPLICABILITY

As will be understood from the foregoing discussion, the presentinvention serves to reduce HC and NOx in exhaust gases released from acombustion chamber when a catalyst is still in its unheated state,resulting in an improvement in emissions, and significantly promotecatalyst quick light-off operation as a result of an increase in exhaustgas temperature. The invention is particularly suited for application toa direct injection engine mounted on a motor vehicle, for example.

What is claimed is:
 1. A control device for a direct injection enginehaving a catalyst provided in an exhaust passage for converting exhaustgases and an injector for injecting fuel directly into a combustionchamber, the control device comprising: a temperature state identifierfor judging the temperature state of the catalyst; and a fuel injectioncontroller for controlling fuel injection from the injector; wherein thefuel injection controller controls the injector based on judgmentresults of the temperature state identifier in such a way that theinjector makes at least two-step split injection during a period from anintake stroke to an ignition point including a later injection cycleperformed in a middle portion of a compression stroke or later and anearlier injection cycle performed prior to the later injection cycle atleast in a low-load range of the engine when the catalyst is in itsunheated state, in which its temperature is lower than its activationtemperature, and the later injection cycle and earlier injection cycleinject fuel which contribute to a main combustion during a maincombustion period in which approximately 10% to 90% by mass of theinjected fuel is burned from start to end of combustion occurring in thecombustion chamber.
 2. A control device for a direct injection engineaccording to claim 1, wherein the fuel injection controller controls theinjector in such a way that the injector makes the split injectionregardless of whether the engine has already been heated or not but whenthe catalyst is in its unheated state.
 3. A control device for a directinjection engine having a catalyst provided in an exhaust passage forconverting exhaust gases and an injector for injecting fuel directlyinto a combustion chamber, the control device comprising: a temperaturestate identifier for judging the temperature state of the catalyst; anda fuel injection controller for controlling fuel injection from theinjector; wherein the fuel injection controller controls the injectorbased on judgment results of the temperature state identifier in such away that the injector makes at least two-step split injection during aperiod from an intake stroke to an ignition point including a laterinjection cycle performed in a middle portion of a compression stroke orlater and an earlier injection cycle performed prior to the laterinjection cycle at least in a low-load range of the engine when thecatalyst is in its unheated state, in which its temperature is lowerthan its activation temperature, and the later injection cycle andearlier injection cycle inject fuel which contribute to a maincombustion during a main combustion period in which approximately 10% to90% by mass of the injected fuel is burned from start to end ofcombustion occurring in the combustion chamber, wherein the fuelinjection controller controls the injector in such a way that theinjector makes two-step split injection including an earlier injectioncycle performed during the period of an intake stroke and a laterinjection cycle commenced in a middle portion of a compression stroke orlater but not later than 45° before top dead center thereof when thecatalyst is in its unheated state, in which its temperature is lowerthan its activation temperature; and sets the air-fuel ratio in theentire combustion chamber to fall within a range of 13 to 17 and theamount of fuel injected in the earlier injection cycle to fall within arange of about ⅕ to about ⅘ of the total amount of the injected fuel. 4.A control device for a direct injection engine according to claim 3,wherein the amount of fuel injected in the earlier injection cycle ofthe split injection performed when the catalyst is in its unheated stateis such an amount that produces a lean mixture which has a higherair-fuel ratio than the stoichiometric air-fuel ratio and is ensuresflame propagation at least by fuel injected in the later injection cycleand combustion thereof, and the lean air-fuel ratio higher than thestoichiometric air-fuel ratio capable of flame propagation by the fuelinjected in the later injection cycle and combustion thereof produced inthe combustion chamber by the earlier injection cycle alone is set to 85or less.
 5. A control device for a direct injection engine according toclaim 3, wherein the amounts of fuel injected in the earlier injectioncycle and the later injection cycle of the split injection performedwhen the catalyst is in its unheated state are made equal to each other.6. A control device for a direct injection engine according to claim 3,wherein mode of fuel injection from the injector is switched to intakestroke injection or to compression stroke injection when the catalysthas reached its heated state and its temperature has become equal to orhigher than its activation temperature.
 7. A control device for a directinjection engine according to claim 3, further comprising an ignitiontiming controller for retarding an ignition point by a specified amountfrom a minimum spark advance for best torque (MBT) when the splitinjection is performed while the catalyst is in its unheated state.
 8. Acontrol device for a direct injection engine according to claim 7,wherein the ignition point is retarded by a specified amount from theMBT when the catalyst is in its unheated state and then the ignitionpoint is advanced toward the MBT according to an increase in catalysttemperature.
 9. A control device for a direct injection engine accordingto claim 3, wherein the air-fuel ratio in the entire combustion chamberis set to a level equal to or higher than the stoichiometric air-fuelratio when the catalyst is in its unheated state.
 10. A control devicefor a direct injection engine according to claim 9, further comprising:an O₂ sensor whose output varies at the stoichiometric air-fuel ratio;and an injection amount calculator for calculating the amount of fuel tobe injected by feedback control such that the air-fuel ratiosubstantially matches the stoichiometric air-fuel ratio based on theoutput of the O₂ sensor after it has become activated when the catalystis in its unheated state.
 11. A control device for a direct injectionengine according to claim 9, further comprising: an O₂ sensor whoseoutput varies at the stoichiometric air-fuel ratio; and an injectionamount calculator for calculating the amount of fuel to be injected byfeedback control based on the output of the O₂ sensor after it hasbecome activated when the catalyst is in its unheated state; wherein theinjection amount calculator sets the air-fuel ratio at a point offset tothe lean side of the stoichiometric air-fuel ratio by adjusting one ofsuch factors as a delay time of change in a control value applicablewhen the output of the O₂ sensor is inverted, a constant ofproportionality and an integration constant.
 12. A control device for adirect injection engine according to claim 9 or 11, wherein the air-fuelratio in the entire combustion chamber is set to a level higher than thestoichiometric air-fuel ratio when the catalyst is in its unheated stateand then the air-fuel ratio is varied to the rich side with an increasein catalyst temperature.
 13. A control device for a direct injectionengine according to one of claims 1 to 3, further comprising aturbulence enhancer for enhancing turbulence within the combustionchamber when the catalyst is in its unheated state.
 14. A control devicefor a direct injection engine according to one of claims 1 to 3, furthercomprising an engine speed controller for controlling the engine suchthat its idling speed becomes higher when the catalyst is in itsunheated state than after the catalyst has been heated.
 15. A controldevice for a direct injection engine having a catalyst provided in anexhaust passage for converting exhaust gases and an injector forinjecting fuel directly into a combustion chamber, the control devicecomprising: a temperature state identifier for judging the temperaturestate of the catalyst; and a fuel injection controller for controllingfuel injection from the injector; wherein the fuel injection controllercontrols the injector based on judgment results of the temperature stateidentifier in such a way that the injector makes at least two-step splitinjection during a period from an intake stroke to an ignition pointincluding a later injection cycle performed in a middle portion of acompression stroke or later and an earlier injection cycle performedprior to the later injection cycle at least in a low-load range of theengine when the catalyst is in its unheated state, in which itstemperature is lower than its activation temperature, and the laterinjection cycle and earlier injection cycle inject fuel which contributeto a main combustion during a main combustion period in whichapproximately 10% to 90% by mass of the injected fuel is burned fromstart to end of combustion occurring in the combustion chamber, and aload condition detector for sensing engine load conditions, wherein thefuel injection controller controls the injector based on judgmentresults of the temperature state identifier and on sensing results ofthe load condition detector in such a way that the injected injects fuelin a compression stroke to have the engine perform stratified chargecombustion in its low-load range and injects fuel in an intake stroke tohave the engine perform uniform combustion in its high-load range whenthe catalyst is in its heated state, in which its temperature is equalto or higher than its activation temperature, and the injector makestwo-step split injection including an earlier injection cycle performedduring the period of the intake stroke and a later injection cyclecommenced in a middle portion of the compression stroke or later but notlater than 45° before top dead center thereof at last in an low-loadrange of the engine when the catalyst is in its unheated state, in whichits temperature is lower than its activation temperature; and sets theair-fuel ratio in the entire combustion chamber to fall within a rangeof 13 to 17 and the amount of fuel injected in the earlier injectioncycle to fall within a range of about ⅕ to ⅘ of the total amount of theinjected fuel in the split injection.
 16. A control device for a directinjection engine having a catalyst provided in an exhaust passage forconverting exhaust gases and an injector for injecting fuel directlyinto a combustion chamber, the control device comprising: a temperaturestate identifier for judging the temperature state of the catalyst; anda fuel injection controller for controlling fuel injection from theinjector; wherein the fuel injection controller controls the injectorbased on judgment results of the temperature state identifier in such away that the injector makes at least two-step split injection during aperiod from an intake stroke to an ignition point including a laterinjection cycle performed in a middle portion of a compression stroke orlater and an earlier injection cycle performed prior to the laterinjection cycle at least in a low-load range of the engine when thecatalyst is in its unheated state, in which its temperature is lowerthan its activation temperature, and the later injection cycle andearlier injection cycle inject fuel which contribute to a maincombustion during a main combustion period in which approximately 10% to90% by mass of the injected fuel is burned from start to end ofcombustion occurring in the combustion chamber, wherein the fuelinjection controller controls the injector based on judgment results ofthe temperature state identifier in such a way that the injector injectsfuel in an intake stroke to have engine perform uniform combustion whenthe catalyst is in its heated state, in which its temperature is equalto or higher than its activation temperature, and the injector makestwo-step split injection including an earlier injection cycle performedduring the period of the intake stroke and a later injection cyclecommenced in a middle portion of the compression stroke or later but notlater than 45° before top dead center thereof at least in a low-loadrange of the engine when the catalyst is in its unheated state, in whichits temperature is lower than its activation temperature; and sets theair-fuel ratio in the entire combustion chamber to fall within a rangeof 13 to 17 and the amount of fuel injected in the earlier injectioncycle to fall within a range of about ⅕ to about ⅘ of the total amountof the injected fuel in the split injection.
 17. A control device for adirect injection engine according to claim 15 or 16, further comprisingan ignition timing controller for retarding an ignition point by aspecified amount from a minimum spark advance for best torque (MBT). 18.A control device for a direct injection engine according to claim 15 or16, wherein the air-fuel ratio in the entire combustion chamber is setto a level equal to or higher than the stoichiometric air-fuel ratiowhen the catalyst is in its unheated state.
 19. A control device for adirect injection engine according to claim 18, further comprising: an O₂sensor whose output varies at the stoichiometric air-fuel ratio; and aninjection amount calculator for calculating the amount of fuel to beinjected by feedback control such that the air-fuel ratio matches thestoichiometric air-fuel ratio, based on the output of the O₂ sensorafter it has become activated when the catalyst is in its unheatedstate.
 20. A control device for a direct injection engine having acatalyst provided in an exhaust passage for converting exhaust gases andan injector for injecting fuel directly into a combustion chamber, thecontrol device comprising: a temperature state identifier for judgingthe temperature state of the catalyst; and a fuel injection controllerfor controlling fuel injection from the injector; wherein the fuelinjection controller controls the injector based on judgment results ofthe temperature state identifier in such a way that the injector makesat least two-step split injection during a period from an intake stroketo an ignition point including a later injection cycle performed in amiddle portion of a compression stroke or later and an earlier injectioncycle performed prior to the later injection cycle at least in alow-load range of the engine when the catalyst is in its unheated state,in which its temperature is lower than its activation temperature, andthe later injection cycle and the earlier injection cycle inject fuelwhich contributes to maintain combustion during a main combustion periodin which approximately 10% to 90% by mass of the injected fuel is burnedfrom state to end of a combustion occurring in the combustion chamber;and sets the air-fuel ratio in the entire combustion chamber to fallwithin a range of 13 to 17, and such an amount of fuel injected in theearlier injection cycle of the split injection performed when thecatalyst is in its unheated state that produces a lean mixture having anair-fuel ratio in the combustion chamber obtained by the earlierinjection cycle alone which is higher than a flammability limit air fuelratio and is 85 or less, and ensures flame propagation at least by fuelinjected in the later injection cycle and combustion thereof.
 21. Acontrol device for a direct injection engine according to claim 20,wherein the amount of fuel injected in the earlier injection cycle isequal or lower than that of fuel injected in the later injection cycle.22. A control device for a direct injection engine according to claim20, wherein the air-fuel ratio in the entire combustion chamber is setto a level higher than the stoichiometric air-fuel ratio but not higherthan 17 when the catalyst is in its unheated state.
 23. A control devicefor a direct injection engine according to claim 20, wherein theair-fuel ratio in the entire combustion chamber is set to a level equalto or higher than the stoichiometric air-fuel ratio when the catalyst isin its unheated state.
 24. A control device for a direct injectionengine according to claim 23, further comprising; an O₂ sensor whoseoutput varies at the stoichiometric air-fuel ratio; and an injectionamount calculator for calculating the amount of fuel to be injected byfeedback control wherein the air-fuel ratio in the entire combustionchamber is set to a level higher than the stoichiometric air-fuel ratiowhen the catalyst is in its unheated state; and the injection amountcalculator calculates the amount of fuel such that the air-fuel ratiomatches the stoichiometric air-fuel ratio based on the output of the O₂sensor after it has become activated when the catalyst is in itsunheated state.
 25. A control device for a direct injection engineaccording to claim 23, further comprising: an O₂ sensor whose outputvaries at the stoichiometric air-fuel ratio; and an injection amountcalculator for calculating the amount of fuel to be injected by feedbackcontrol based on the output of the O₂ sensor after it has becomeactivated when the catalyst is in its unheated state; wherein theinjection amount calculator sets the air-fuel ratio at a point offset tothe lean side of the stoichiometric air-fuel ratio by adjusting one ofsuch factors as a delay time of change in a control value applicablewhen the output of the O₂ sensor is inverted, a constant ofproportionality and an integration constant.
 26. A control device for adirect injection engine according to claim 23, wherein the air-fuelratio in the entire combustion chamber is set to a level higher than thestoichiometric air-fuel ratio when the catalyst is in its unheated stateand then the air-fuel ratio is varied to the rich side with an increasein catalyst temperature.
 27. A control device for a direct injectionengine according to claim 20, wherein mode of fuel injection from theinjector is switched to intake stroke injection or to compression strokeinjection when the catalyst has reached its heated state and itstemperature has become equal to or higher than its activationtemperature.
 28. A control device for a direct injection engineaccording to claim 20, further comprising an ignition timing controllerfor retarding an ignition point by a specified amount from a minimumspark advance for best torque (MBT) when the split injection isperformed while the catalyst is in its unheated state.
 29. A controldevice for a direct injection engine according to claim 28, wherein theignition point is retarded by a specified amount from the MBT when thecatalyst is in its unheated state and the ignition point is advancedtoward the MBT according to an increase in catalyst temperature.
 30. Acontrol device for a direct injection engine according to claim 20,further comprising a turbulence enhancer for enhancing turbulence withinthe combustion chamber when the catalyst is in its unheated state.
 31. Acontrol device for a direct injection engine according to claim 20,further comprising an engine speed controller for controlling the enginesuch that its idling speed becomes higher when the catalyst is in itsunheated state than after the catalyst has been heated.